1. Field of the Invention
The present invention relates to a system for recording and retrieving information. More particularly, it relates to an electron beam memory system of the phase transition type wherein the phase transition of a recording medium is effected with an electron beam and wherein the state of the phase transition is detected with an electron beam.
2. Description of the Related Art
As systems for recording and retrieving digital information, there have been known a magnetic disc system, an optical disc system, etc. Any of these systems performs recording and retrieval in such a way that a physical or chemical change is caused in a recording medium by the use of a magnetic flux signal or an optical signal, and that the presence and absence of the physical or chemical change correspond to digital information items "1" and "0."
A system utilizing the phase transition of a recording medium as the physical or chemical change thereof is disclosed in U. S. Pat. No. 3,530,441. This system is such that atoms in the recording medium are moved by heating with a laser beam, so as to generate local phase transition states in the recording medium. However, in case of recording information by the use of light in this manner, the smallest light beam spot which can be converged is of approximately 1 .mu.m in diameter because of the limit of the wavelength of the light, and the limit of the recording density of the system has been 10.sup.8 bits/cm.sup.2.
Meanwhile, an electron beam recording and retrieving method wherein information is recorded and retrieved by employing an electron beam instead of the laser beam is disclosed in the official gazette of Japanese Patent Application Laid-open No. 62845/1983. This method is such that a recording medium which comprises a substrate, a chalcogenide thin film (Se-Ge-based thin film) disposed on the substrate, and an Ag thin film formed on the chalcogenide thin film, is irradiated with the electron beam so as to diffuse Ag into the chalcogenide thin film on the basis of the electron beam exposure, thereby to record information, while the difference between the secondary electron emissivities of exposed parts and unexposed parts is sensed, thereby to read the information. Since, however, the Ag thin film disposed on the chalcogenide thin film has the disadvantage of liability to oxidation in the atmospheric air, the method involves the problem that the retrieval efficiency of information lowers due to the oxidation of the surface of the Ag thin film. Another problem is that, since information is recorded by exploiting the diffusion of Ag into the chalcogenide thin film as based on the electron beam exposure, it cannot be rewritten.